Oldtown is the second largest and most populated city in the Seven Kingdoms, and by far the oldest major city in Westeros, dating back to the time of the First Men. It is located in the far southwest of the continent, at the mouth of the River Honeywine where it opens into Whispering Sound and the Sunset Sea beyond.
The major highway known as the Roseroad begins at Oldtown, then runs northeast past Highgarden before finally reaching King's Landing on the east coast. The Searoad also forks off from the Roseroad at Highgarden, going northwest so travelers from Oldtown can reach Lannisport.
Oldtown is the seat of House Hightower, who hold fealty to Bronn's House of Highgarden, from the eponymous Hightower. The city is also notable as the site of the Citadel, home of the order of maesters, as well as the Starry Sept, the former center of the Faith of the Seven in Westeros (it has since been supplanted by the Great Sept of Baelor in King's Landing).
To sneak into Dorne, Jaime and Bronn board the ship of a Pentoshi Merchant captain headed south from King's Landing. Jaime bribes him to let them off at the coast of Dorne on a rowing boat, as the ship continues on to Planky Town before heading to its final destination at Oldtown. The captain, however, later tries to sell information about Jaime's arrival to Obara Sand, but gets killed.
Sam and Gilly continue on a ship bound for Oldtown. Gilly says the ship's captain told her that Oldtown is considered the most beautiful city in all of Westeros.
Oldtown officially supplants King's Landing as the most populated city, after the Battle of King's Landing.
In the books
In the A Song of Ice and Fire novels, Oldtown was the largest city in Westeros before the Targaryen conquest. It survived for so long through canny politicking by the ruling Hightowers, who trace their ancestry back to the First Men. In time, through patronage of local kings of the First Men, the Order of Maesters was founded in Oldtown, a guild of scholars and healers, with their headquarters located at the Citadel in the city.
When the Andals invaded, the Hightowers, then kings in their own right, welcomed them openly and swore fealty to them as Kings of the Reach. Oldtown's pre-eminence was confirmed when the Faith of the Seven built the Starry Sept there.
During the invasion of Aegon the Conqueror, the High Septon of the time prayed and fasted for seven days and seven nights on whether the Hightowers should resist Aegon's army. He reportedly received a vision from the Crone showing Oldtown's destruction should they attempt this. At his recommendation, the then-Lord Hightower threw open the city gates and welcomed Aegon into the city with open arms. Aegon dated the first day of his reign to his welcome into Oldtown.
Oldtown is almost as large and populous as King's Landing, but is far cleaner, more beautiful and more efficiently run. King's Landing was rapidly constructed within the first century of rule by the Targaryen dynasty. This rapid expansion didn't keep pace with organized "city-planning", resulting in numerous slums forming such as the district known as Flea Bottom. Many of these slum districts are poorly organized, ugly, and filthy because their placement wasn't thought out during the rush to slap down new buildings. By contrast, Oldtown is thousands of years old and grew only in incremental steps which were well planned out beforehand. The result is that Oldtown is a much better organized and beautiful city without the squalid slums of King's Landing, while being only slightly smaller than the capital itself. Oldtown also has its own City Watch, separate from the City Watch of King's Landing.
Oldtown is famed for its meandering, cobbled streets and many fine inns and septs. It has an immense port which plays host to ships from across the Seven Kingdoms, the Free Cities, and even the distant Summer Islands. The port and the entire city is dominated by the Hightower, a massive, seven-sided, stepped lighthouse rising from Battle Island. More than 800 feet high, it is the tallest structure in Westeros, higher even than the Wall. It serves as House Hightower's keep, as well as a lighthouse.
Oldtown has had the misfortune, as a large city, of being struck twice by major plagues in the past century. In 209 AC, a great pandemic known as the Great Spring Sickness swept across all of the Seven Kingdoms. It killed tens of thousands, particularly in the major cities. It was bad in Lannisport, worse in Oldtown, but worst of all in King's Landing, where two out of every five persons died. The death count wasn't quite as severe in Oldtown, but still high. Only one generation later, during Pycelle's youth (220s to 230s AC), Oldtown itself was hit by an even worse disease, the "grey plague" (a more deadly cousin of Greyscale). According to Pycelle, the outbreak killed up to half of Oldtown's population, and three quarters of the maesters in the Citadel. The only reason it didn't become another, even more destructive pandemic is because Lord Quenton Hightower ordered the gates of the city sealed and all ships in the harbor burned to prevent the disease being spread further. While this did prevent another pandemic, many trapped inside the city's quarantine caught the disease from those who were already sick, and perished. After the quarantine ended, the survivors dragged Lord Quenton and his young son from their horses and slit their throats. By the time of the War of the Five Kings, some seventy years later, Oldtown's population is slightly smaller than that of King's Landing, but it may have been larger over a century ago, before the double-blow of these two epidemics, and when King's Landing was smaller.
No one knows exactly how old the city really is, as it's true origins stretch back before history and into oral tradition. There was always a major settlement at the mouth of the Honeywine (though it did grow and expand considerably over the millennia). The First Men apparently founded the original town that later became a city relatively soon after they first began migrating into Westeros 12,000 years ago, across the nearby land-bridge at the time linking Dorne and Essos. A few maesters argue that there may have been even earlier settlements at the mouth of the Honeywine that the First Men built over - perhaps a trading post maintained by anyone from the Valyrians to the Rhoynar to the Summer Islanders (or a combination of them), where their fishing fleets could take on fresh water and perhaps even trade with the non-human races that dominated Westeros's wild forests at the time (the Children of the Forest and the Giants). The base of the Hightower and underground levels seem to be the remnant of some ancient structure, made of an oily black stone found in a few other ruins across the world. The evidence, however, is very sketchy and inconclusive.
Oldtown remains aloof from the War of the Five Kings. In the fourth novel, Euron launches a massive raid along the coast, conquering the Shield Islands and parts of the Arbor, then trying to blockade the mouth of the Honeywine. An ironborn attack on the city's harbor is repulsed by the city's defenders, but Oldtown remains under threat from the ironborn.
Oldtown bears many similarities to real-life Alexandria. This Egyptian city - founded in the 4th Century BCE by Alexander the Great - was (and still is) a major port on the Mediterranean Sea, receiving ships and cargoes from all over the world. The Citadel of Oldtown, with its enormous library and status as a place of learning, is very similar to the famed Library of Alexandria, the pre-eminent educational center of its time. Perhaps most strongly, the Hightower of Oldtown is a clear parallel to Alexandria's famous lighthouse, one of the Seven Wonders of the Ancient World.
Old Oak · Red Lake · Goldengrove · Bitterbridge · Tumbleton · Grassy Vale · Cider Hall · Ashford · Horn Hill · Brightwater Keep · Bandallon · Honeyholt · Oldtown (Citadel · Hightower · Starry Sept) · Uplands · Blackcrown · Three Towers · Sunflower Hall · Arbor